1. Field of the Invention
This invention relates to a magnetic pipe locator having three axes for determining the magnitude and direction of the magnetic field generated by underground pipes having a magnetic signature.
2. Description of Prior Art
Pipes buried below the surface of the earth require periodic service and/or repair. Damaged pipes must be located and excavated in order to receive such service. Location of underground pipes is a traditionally difficult task, especially in areas containing large networks of underground pipes, wires and other buried objects.
Underground pipelines are generally constructed of either metal or, commonly, plastic such as polyethylene and polyvinyl chloride. Plastic is a preferred pipe material because it is durable, inexpensive, lightweight, inert, easy to manufacture and easy to install. However, plastic pipe, if not treated, is undetectable with traditional underground pipeline location methods.
Plastic pipe may be treated to impart a detectable feature into the plastic and allow for detection for repair and/or service. Goodman, U.S. Pat. No. 5,036,210, and Goodman, U.S. Pat. No. 5,354,521, are methods of making a magnetically detectable plastic pipe by distributing magnetic particles within the wall of the plastic pipe during the extrusion of the pipe. The ""210 patent and the ""521 patent teach that distinct magnetic signatures may be created by changing the polarity of predetermined portions of the magnetic particles within the pipe.
Prior art magnetic pipe locators do not have the capability to discriminate against other buried magnetic objects, called clutter, in the vicinity of the target pipe. A pipe locator is desired which can identify a pipe having a unique pipe signature, such as the signature created by the pipes taught by the ""210 patent and the ""521 patent. The need for a pipe locator with the capability to discriminate against clutter is created by buried magnetic plastic pipe which may be as magnetic and sometimes less magnetic than other buried magnetic objects.
Prior art pipe locators commonly use a single vertical magnetic field sensor. One problem of the prior art pipe locators in recognizing a unique magnetic signature of a pipeline, such as a spiral signature, is that the signature may have a rather long wavelength, for instance 20 feet. This means that the operator of a hand held locator must traverse a considerable distance before gathering enough information for the pipe spiral signature to be recognized. A way of solving this is to add memory and processing so that a map of a region can be displayed after capturing the original data.
Prior art locators add magnetic polarity detection to enable the changing direction of the spiral pipe magnetization to be detected. Prior art locators include single axis magnetic sensors.
One prior art pipe locator, taught by Goodman, U.S. Pat. No. 5,321,361, was developed to extend the performance of the existing technology in the ability to detect and display magnetic field direction as well as magnitude. This locator employed two fluxgate sensors arranged in a gradient configuration and a vertical position and balanced so that the locator would not respond to rotations in the Earth""s uniform 50,000 gamma magnetic field. This locator was equipped with a tone generator to indicate the presence of magnetic objects and a meter to indicate whether the detected field was up or down. The threshold detection capability of this locator was is about 50 gammas, enabling detection of 5% and 7% barium and strontium plastic pipes down to depths of 5 feet. This locator also includes a visual LCD bar graph display of the output signal of both the amplitude and polarity of the output. The locator also included a range changing feature important when signals with large magnitude variation are encountered, typical when both magnetic pipe and large amounts of pipe are present.
It is one object of this invention to provide a new and improved magnetic pipe locator for detecting magnetically detectable plastic pipe or other sources of magnetic fields from a distance.
It is an object of this invention to provide an apparatus that detects both the magnitude and direction of an underground magnetic field.
It is another object of this invention to provide a magnetic pipe locator that can detect a specific magnetic pattern despite the presence of clutter.
It is yet another object of this invention to provide a magnetic pipe locator that is insensitive to the earth""s magnetic field.
It is still another object of this invention to add memory and processing to a magnetic pipe locator so that a map of a region can be displayed after capturing the relevant data.
These and other objects of this invention are achieved with a magnetic pipe locator having three axis sensing capability. Prior art pipe locators generally contain sensors in one or possibly two axes. The use of three orthogonal sensors enables both the magnitude and direction of the surface magnetic field to be measured. Since the spiral pipe magnetization pattern is essentially a constant magnitude change in the direction of pipe magnetization along its length, the use of three sensors provides enhanced ability to recognize this pattern in the presence of clutter. Most clutter signals encountered show large magnitude and direction changes in magnetization. Generally one axis will be minimally affected by clutter even if the other two axes are obscured to a degree.
The pipe locator according to this invention is also capable of recognizing unique magnetic pipeline signatures, such as spiral signatures. Because of the memory and processing capabilities of the multi-axis locator according to the present invention, a map of a region can be displayed after capturing the original data. This allows the multi-axis pipe locator operator to identify specific pipe signatures within the inspected region.
The multi-axis pipe locator according to this invention has the capability of producing maps of the magnetic field intensity over an area. This affords the possibility of performing sophisticated pattern recognition analysis of the surface magnetic field. Since magnetized plastic pipe often produces a distinct magnetic signature, the multi-axis pipe locator according to this invention is potentially much more capable of recognizing the pipe in the presence of clutter. It employs discrete sensors and is capable of producing real time maps of magnetic pipe signatures.